Method of separating fused products containing partially vitreous and partially nonvitreous materials



BLACK 2,257,325

METHOD OF SEPARATING FUSED PRODUCTS CONTAINING PARTIALLY VITREOUS ANDPARTIALLY NONVITREOUS MATERIALS Filed Jan. 50, 1939 FUSION PRODUCTscALPER SCREEN 3 U CRYSTHLLIHE TUBE MILL SCREEN II N 5 Ir w r S STORAGEFOR CRYSTHLLIHEPRODOCT aLnss To STORAGE INVENTOR LEROY G. BLACK MfiATTORNE Y5 Patented Sept. 30, 1941 METHOD OF. SEPARA'I'ING FUSED PROD-UOTS CONTAINING PARTIALLY VITREOU S AND PARTIALLY NONVITREOUS MATE-RIALS Leroy G. Black, Trona, Calif., assignor to-American Potash &Chemical Corporation, Trona,

Calif., a corporation of Delaware Application January 30, 1939, SerialNo. 253,524

3 Claims. (01. 83-94) This invention relates to a process of separatingfused products, which consist partially of "vitreous and partially ofnon-vitreous material by' grinding and classification.

Certain fused products consist of intimate mixtures of vitreous materialand non-vitreous material. It is frequently desirable to separate thesetwo constituents. Thus, for example, in

the dehydration of hydrated borates, a fused mass is produced which uponcooling may consist partially of vitreous borax, known as borax glass,and partially of granular or crystalline anhydrous sodium tetraborate,NazBrOi.

The granular or crystalline material firmly adheres to the vitreous orglass portion. Heretofore, no effort has been made to seperate the twoforms and the mixed product has been ground completelyto produceanhydrous sodium tetraborate for the market. for both types of products,and superior products 7 can be produced by separating theseconstituents.

It is therefore, the general object of thepresent invention to provide amethod by which 'f used products can be readily and economicallyseparated into' their vitreous and non-vitreous constltuents.

I have discovered that fused products, such the fused mixture ofvitreous borax glass with the granular crystalline anhydrous sodiumtetraborate, may be readily and economically sep arated one from theother by a process of selectively grinding the material. I have foundthat by proper selective grinding it is possible to comminute granular.or crystalline material to a finer size than the vitreous or glassconstituent, whereby the two constituents may be readily separated onefrom the other. For example, the process of the present invention may beapplied to the product of United States Patents Nos 1,964,902 and2,064,337. The product produced as described in such patents consistsprimarily of anhydrous sodium tetraborate in the form of crystals, andsuch a product isfoundto possess many advantages in industries requiringa raw material having a boron content; There 3 remains, however, alimited but definite demand for anhydrous borax in the vitreous state,or borax glass. Although the product of the above patents consistsprincipally of crystals of anhydrous borax, it may contain a limitedquantity .1

producing such material, the material is run into There are, however,separate markets of crystalline matter.

open molds and allowed to. crystallize. Crystallization is enhanced byslow cooling. The more rapid the cooling, the more of the vitreousmaterial produced. In any case, a thin layer of material. next to thewalls of the mold andalso a thin skin on the upper surface of the massin the mold usually will be cooled too quickly to allow forcrystallization, and will, therefore, be in the form of borax'glass.Increased quantities of glass may be produced in the process when thefused material is heated to temperatures appreciably above the fusionpoint; that is, when the fused material becomes strongly superheated,aswhen it remainswithin the fusion furnace for sometime. The proportionof glass may be varied in this way to as much as 25%. With suchproducts, as also with similar fusionproducts of other substances, theexterior vitreous matter is very intimately associated with the granularor crystalline portions.

It is one of the objects of the present invention to provide a simpleprocess by which such products may be treated, so that the vitreousandnon-vitreous portions thereof are readily separable and preferably bothattained as marketableproducts.

I have discovered that such mixtures of vitreous and crystallinefractions may be eifectively separated into their component parts by aselective or difierential grinding process which comminutes'thecrystalline fraction to a greater extent than the" vitreous fraction,followed by a step to separate the two fractions according to size. Thegrinding may be controlled so that the crystalline matter is fracturedand broken down into small grains while the vitreous matter is caused toremainmostly as larger particles, which can be segregated from the finerparticles The crystalline fraction has'been found to be suflicientlyfriable that individual grains or crystal fragments maybe separated fromthe vitreous matter by a grinding process. In fact, if desired, thecrystalline matter may be reduced essentially to a powder withoutdestroying the larger vitreous particles. By the present invention, ithas been found possible to producea product which is composed almostentirely of the vitreousparticles, being free of appreciable quantitiesof crystalline matter. Furthermore, the net recovery of vitreousparticles can be made as high as -90%; and the process may be so carriedout that two products of different sizes are attained.

' The process of the present invention is based primarilyupon the use ofabrasion or attrition grinding as opposed to ordinary continuouspressure or blow or impact types of pulverizers. Preferably, thematerial to be processed is first reduced to approximately the size of ahen-egg and then subjected to a polishing type of grinding of suchintensity and nature as to break down the crystalline fragments intoindividual grains and to rub the crystalline particles from the vitreousfragments to which they adhere, and in that manner reduce thecrystalline material to a size appreciably smaller than thatof-thegreater part of the vitreous matter. Thereby I produce a ground mixturewhich contains crystalline and vitreous matter as particles ofappreciably different size, and separation of the two smaller than thevitreous particles.

forms is a simple matter of size segregation. 1 Screening or airclassification, for example, can

be used to obtain the two materials as separate tween each stage, theintensity of the attrition grinding being decreased and the duration oftreatment increased in successive stages. 'Stage grinding is preferredto permit the grinding characteristics to be varied in accordance withchanges in the reaction of the material to grinding, as the crystallinefraction becomes pulverized and reduced in quantity. a r

Preferably, the first stage of attrition grinding is carried out toreduce the material to a maximum size of about to in diameter. The glassparticles will be fairly free of adhering crystalline material andalarge part of said 7 crystalline material will have been reduced to asize readily separable from the glass by screening. This result isattained by using choke feeding provided by using a circulating loadwith an attrition type disintegrator, whereby there is maintained alarge amount of granular material in the mill during the grindingaction, which serves to'cushion the action of the grinding, medium onthe vitreous particles. The duration of grind.- ing in the first stageis preferably maderelatively short, and a combination of grindingintensity and duration of treatment is selected so that 'all of the rawmaterial is reduced at least to about A." to /2" size fairly quickly. Alarge part of the crystalline matter becomes pulverized during the timeneeded to reduce all of the material to a size of about A" to and alsothe greater part of the crystalline matter embedded in vitreous surfacesis unlocked and brokenaway.

' The product from the first stage of abrasion grinding is passed over ascreening device for the separation of the fines and coarser particles.This separation may be made at various screen sizes, and I have foundthat a cut made at about 14 to 16 mesh will segregate the large glassparticles as oversize. will be a small percentage of the originalcrystalline fraction. The undersize from the screening process willcontain a high percentage of crystalline particles, and the sizedistribution thereof is suitable for a commercial product.

Following the screening operation, I subject the oversize particles tofurther abrasion grinding of less intensity and greater duration thanthat employed in the initial. treatment. This final grinding must avoid,as before, subjecting Also, contained in the latter the vitreousparticles to sufficient abrasion to reduce them to a fine particle size,and in this case there are not sufficient crystalline particlesavailable to cushion the grinding media and reduce their action on theparticles of vitreous matter. I have found that only little additionalgrinding force is required to reduce essentially all of the crystallineparticles to a size much Furthermore, during the reduction or grindingof the crystalline matter in this manner only a small percentage of thevitreous matter is reduced to a particle size comparable with that ofthe crystalline fraction. Therefore, in this final stage or stages ofthe attrition grinding, I employ very "light grinding media and permitsuch light media to act on the crystalline matter for an extended periodsufficient to reduce all of the latter to a relatively smaller size.

Following the final stage of grinding, the product is again subjected toa size separation operation. This may consist of a screening step or ofsomeother method of classification, such as air classification, forexample. The exact size at which the separation-is to bemade will varywith the particular type ofgrinding used. In the final grinding thevitreous matter will be considerably reduced in size although theparticle size rang will be much greater than that of the granular orcrystalline fraction. I have found, for example, that excellentseparation of granular material and good glass recovery are obtainedwhen the separation after the final stage of grinding is'made on a20-mesh screen The smallest separated vitreous grains are just plusZO-mesh' size, while particles as large as A may exist in the mixture.The crystalline fraction will be entirely minus ZO-mesh, but ,the actualsize pattern will vary over a considerable range below 20-mesh. r

Many types of abrasion and attrition mills are capable of producing, thedesired selective 'comminution action, it being necessary only to selectequipment which Is exerts a grinding action whose intensity willgranulate or powder the crystalline particles but not the vitreousmatter.

.Not only are the mills to which the term attrition mills is usuallyapplied applicable, but also mills of the impact or direct blow typemay'often be modified to exert "predominantly attritive and abrasiveaction on mixtures containing vitreous they willgrind with an intensityinsuflicient to powder too much of the vitreous matter during such chokecrushing.

As an example of my invention, I will describe its use for theseparation of vitreous and crystalline anhydrous sodium tetraborate, theapparatus used being that set forth diagrammatically in Figure 1.

Referring now to the drawing, cooled masses of molten Na2B4O7 cast inmolds and produced in accordance with the patents previously cited, arefirst subjected to a preliminary coarse crushing difierential grindingstages of the invention are applicable to best advantage when the rawmaterial to be treated consists of small-sized lumps,

though the sizing for this purpose may produce particlesboth somewhatlarger and smaller than the preferredsize. Any of a large variety ofdisintegrators are useful in this service, and I use a spiked rollcrusher comprisingtwo rolls rotating lnopposite directions. Thiscrushing is effected with free feeding of the mill to keep the quantityoffines to a minimum.

The preliminary crushedproduct is transferred from the rollcrusher to arod mill operated under choke feeding conditions and in conjunction witha scalping screen from which oversize product is partly re-circulatedthrough the mill. A..few

relatively light-weight rods, which may be hollow if desired, are. usedin the mill, the size, weight, and number of rods being governed by thesize of the mill, rate of feed, etc. The criteria determining the designof the mill is that the grinding action must be primarily of theabrasionor attrition type. Light-weight rods grind with less shock or impact,thus preserving the solid particles of glass. I employ the well-knownmethod re-circulating the oversize particles. I prefer this method, asin that way crystalline material ground to the desired fine size issegregated quickly and is not subjected to excessive grinding, whichwould powder it. The rods used in the mill should be of a size onlysufficient to break up the crystalline matter and not suflicient topowder the glass in the time available.

The exact particle size at which the separation is made on the screen isnot critical but may be varied within a certain general range, as thevitreous particles will be mostly of a size appreciably larger than thebulk of the pulverized crystalline matter. I prefer to operate theprocess so that all of the granular material will pass a 14-meshvibrating screen.

I have found that a large part of the crystalline matter will beunlocked from the vitreous matter, with which it is associated, in therod mill. Some of the glass particles will retain adhering crystallinematter. Complete removal of the remaining crystalline matter is thenaccomplished by further crushing or pulverizing. This result is obtainedby subjecting a part of the screen oversize to a further attritiongrinding of longer duration and lower intensity than in the choke-fedrod mill. I do this in a ball mill carrying a light load of small balls.Here again, I desire to have the glass particles broken up as little aspossible, the size and number of the balls used being chosen to thatend. During such attrition grinding by the glass particles, only a minorportion of the glass is pulverized, though the glass particles may beground to a size smaller than that produced in the rod mill. Also,crystalline matter adhering to glass particles will be rubbed orpolished off in the ball mill, leaving a product containing most of theoriginal glass in the form of particles of appreciable size while thecrystalline matter is all present in a much smaller size. I haveproduced a fine product containing most of the glass in the form ofparticles greater than about ZO-mesh in size and the crystalline matteras particles of size less than 20-mesh.

The recovery of crystal-free glass depends upon the number of stages ofgrinding used as the type, intensity, and duration of the grindingshould be varied with the ratio of glass to crys- 'talline matter andwith the size of the particles. --For apra'ctical arrangementwhich givesgood recovery; combined with good capacity, I 'have foundthejarrange'znent shown and described to be satisfactory. f i I While Iprefer to carry out my grinding process in stages, after the preliminarycoarse crushing} asin that'way alarger capacity is obtained,

theentire process may be performed in a single 'mill, such as fthe firstrod mill described. Such a mill must then be designed to have a grindingaction 'which will remove even the intimately associated crystallinematter without destruction of the glassparticles.

The greater part of the friable matte'rimay be comminuted to a Sizewhich is easily-separated from the vitreous particles bya'grindingfor'ce which in the absence ofithe large amount of ungroundfriable matter would damagethe" vitreous particles, and in a multi-stagegrinding" process such a force can be usedin the initial fine grinding.In a single stage, of course, only one degree of grinding can be usedand this must not be sufficient to destroy excessive quantities ofglass.

My invention is applicable to a considerable range and variety ofconditions, and I will set forth the following example of oneapplication thereof to the grinding of ingots of crystalline anhydrousNa2B4O1 containing borax glass. For the initial stage of selectivegrinding a rod mill 3 feet by 8 feet carrying 10 to 22 rods 8 feet longand 3 inches in diameter, and rotating at about 30 R. P. M., was used.The range given for the number of rods permits variations in the millingrate and is necessary also as the rods required for a given rate varywith the composition of the crude mixed product. With this mill amilling rate of 2.5 to 6 tons per hour was possible, the rate dependingupon the rod charge, while the circulating load was held uniform atabout 20 tons per hour. The mill discharge was continuously passed overa Tyler Hummer 4 feet by 8 feet scalping screen having openingscorresponding approximately to i l-mesh U. S. Standard. In preparingonly a limited quantity of the glass, free of crystalline particles,less than the total quantity available, I continuously bled off about0.25 ton per hour of a mixed product containing about of glass. Thisbleed-01f was transferred directly to a tube mill 16 inches in diameterand 12 feet long, carrying a 500 pound charge of /2 inch to 2% inchsteel balls. A 14-mesh scalping screen received the mill discharge. Theoversize material from this latter screen consisted of a productcontaining about 98% of glass and 2% or less of remaining crystallinematter, the recovery of glass being substantially 84%.

While the particular process herein described is well adapted to carryout the objects of the present invention, it is to be understood thatvarious modifications and changes may be made, and the inventionincludes all such modifications and changes as come within the scope ofthe appended claims.

I claim:

1. The process of separating the vitreous and granular forms of sodiumtetraborate from intimately associated admixtures of said forms whichcomprises introducing relatively coarse particles of the mixed materialinto a choke fed rod mill operated to have a difierential grindingaction affecting the granular portion to a greater extent than thevitreous portion, continuously separating the comminuted particles fromthe oversized material and treating at least a part of the latter in achoke fed. ball mill operated to reduce the'greater part of theremaining granular material to a finer size while preserving the greaterpart of the vitreous material as particles of larger size;

2. 'I'he'process of separating the vitreous and granular forms of sodiumtetraborate from intimately associated admixtures of said forms whichcomprises introducing the admixture into a rod mill, subjecting the sametherein under choke feeding conditions to a predominantly abrasion orattrition grinding of intensity sufficient to pulverize a large part ofthe granular material while retaining a substantial part of the vitreousmaterial in the form of particles of larger size, separating andrecovering the pulverized material, grinding at least a part of theremaining larger particles by an abrasion or cient duration to pulverizethe remaining granular material while leaving the greater part of thevitreous material as particles of larger'size, and separating thepulverized material'from the larger vitreous particles. i v

3. The process of separating the vitreous and granular forms of sodiumtetraborate from intimately associated admixtures of said forms, whichcomprises introducing relatively coarse particles of said admixture intoa choke fed'rod mill operated to have a difierential grinding actionaffecting the granular portion to agreater extent than the vitreousfraction, continuing the grinding until the crystalline material isessentially pulverized while preserving the vitreous material asparticles of larger size, separating the crystalline particles from thelarger glass particles'and recovering both products.

LEROY G. BLACVKQ.

